Melt-blown nonwoven fabric is produced by ejecting a filament from spinning nozzles, spraying heated compressed air to the filaments to attenuate the filaments and cause self-fusion, depositing the thus ejected fiber on a collector apparatus to produce the nonwoven fabric. The fiber constituting the melt-blown nonwoven fabric is attenuated by spraying the compressed air and, therefore, oriented crystallization is less likely to occur and the as-produced nonwoven web may experience troubles such as deformation and breakage if it is further processed at a high temperature or used with no further processing. To avoid such troubles, it is important that the as-produced nonwoven web is imparted with thermal size stability, and a typical stabilization means is crystallization treatment by heating.
Typical conventional methods used for thermal crystallization of the nonwoven fabric include thermal compression by heated calendar rolls or embossed rolls, heat treatment by high-temperature heated fluid such as hot air or steam, and heat treatment by infrared heater. Of those heat treatment methods, the thermal compression suffers from the problems of poor workability due to irregular width shrinkage or wrinkling of the nonwoven web. That method also suffers from the problem of a drastic decrease in the gas flow rate due to the collapse of the sheet and, hence, increase in the density after the heat treatment.
In melt-blown nonwoven fabrics, heat treatment is generally conducted under tension by heating the nonwoven fabric with a high-temperature heated fluid or an infrared heater while holding its opposite ends by pins, clips, or other tenter device.
An exemplary heat treatment apparatus that has been proposed is an apparatus wherein consistent heating of the nonwoven web in its transverse direction has been realized by temperature control using a temperature sensor while holding the opposite ends of the nonwoven fabric by using a tenter device (see Japanese Unexamined Patent Publication (Kokai) No. 2002-18970). However, it was only the opposite ends of the nonwoven web that were held by the tenter device in the heat treatment conducted by that heat treatment apparatus, and there has been a problem that the nonwoven fabric frequently suffered from poor texture and waviness of the sheet (nonwoven web) due to partial sheet (nonwoven web) shrinkage with the progress of fiber fusion and inconsistent unit weight.
Also proposed are a method of processing a nonwoven web and a processing apparatus, wherein the nonwoven web is sandwiched between a punched endless belt and a fiber-conveying endless belt and hot air is ejected from the interior side to the exterior side of the punched endless belt to thereby heat the fiber in the hot air-penetrating regions (see Japanese Unexamined Patent Publication (Kokai) No. 2011-219873). However, a punched belt is used in the proposed processing apparatus for penetration of the hot air through the nonwoven web, and this in turn means that the hot air does not penetrate in the non-punched area of the belt. This resulted in the problems that the heat treatment of the fiber is likely to become inconsistent and, also, that the pattern of the punched area is transferred to the nonwoven web. In addition, due to the use of a metal belt having low ability to follow the nonwoven web, areas with insufficient holding is likely to be present in the nonwoven web, resulting in the problem that fiber shrinkage in such an area results in the poor texture.
As described above, there has been no method of producing a melt-blown nonwoven fabric that can impart sufficient thermal size stability to the nonwoven fabric without causing loss of texture or generation of waviness of the nonwoven web or transfer of the belt pattern to the nonwoven web that resulted in the surface unevenness of the nonwoven web in the heat treatment step.
Currently, the nonwoven fabrics are used in various industrial applications such as filters, abrasive cloth, and battery separators. Of those, the performance generally required for the nonwoven fabric in the use for a battery separator is the ability of separating the electrodes and preventing short-circuiting and the ability to retain the electrolyte. In a secondary battery, the nonwoven fabric used for the separator should allow passage of the gas generated by the reaction of the electrode.
Batteries are recently used in wider variety of environments with the progress in the development of various portable equipment and installed sensor and measuring instrument and, in some applications, there is a demand for batteries capable of fulfilling their performance even in the severe environment at high temperature under high impact load.
In such a situation, a separator prepared by using an annealed melt-blown nonwoven fabric of polyphenylene sulfide resin has been proposed for use as a separator in coin-type batteries (see Japanese Unexamined Patent Publication (Kokai) No. 2004-047280), and that procedure enables improvement in heat resistance as well as shrinkage resistance of the nonwoven fabric and production of a battery separator that is free from melting and deformation even at a high temperature. However, the separator of that proposal is associated with the risk of separator breakage and electrode damage in the application where impact load is applied. In addition, annealed nonwoven fabric suffers from the problems of increased variation in the thickness of the nonwoven fabric and gap generation between the electrode and the separator which may invite loss of liquid retention.
In view of such situation, a battery separator comprising a laminate of melt-blown polyphenylene sulfide nonwoven fabrics has been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 2002-343329). That procedure certainly enabled decrease in the thickness of the separator with reduced variation in the thickness thereby improving close contact between the electrode and the separator. However, that proposal was associated with the problem of reduced liquid retention, namely, loss of the merit inherent to the nonwoven fabric and, also, with the problem of mutual adhesion of the fibers resulting in the reduced surface area of the fiber and, hence, in the risk of reduced liquid retention after prolonged use at a high temperature.
As described above, there has been no conventional battery separator that can retain sufficient discharge characteristic without experiencing troubles even when it is used for a long time in high temperature environment where impact load is applied.
In view of the situation as described above, it could be helpful to provide a melt-blown nonwoven fabric having good texture with no waviness or surface unevenness as well as excellent thermal size stability that does not experience drastic decrease in the gas flow rate in the heat treatment. It could also be helpful to provide a method of producing such melt-blown nonwoven fabric.
It could still further be helpful to provide a nonwoven fabric battery separator that exhibits excellent contact with the electrode and is well adapted for use under high temperature environment where impact load is applied without undergoing an increase in the density in the pressure application treatment.